In HVAC applications and beyond, plate heat exchangers can help save money and increase energy efficiency.
Plate heat exchangers (PHE) effectively heat process streams with steam or hot water and cool product streams with tower, city or seawater. The plate heat exchanger provides an excellent means of achieving high efficiency in heat recovery. Full counter current flow and its ability to produce high heat transfer coefficients enable it to produce a close end temperature difference.

Some PHEs can achieve 97 percent heat recovery even at temperatures as low as 2 degrees F. This is important to plants with sources of low-grade or poor quality heat. There is nothing new about heat recovery. In the past, many processors regarded it as not worth the capital expenditure. However, with today's high energy costs, it is not merely considered but desirable. It is a necessity.

The Heating/Cooling Process

In many applications, an immediate savings can be accomplished with an existing system by adding a PHE. Other processes can be made more economical by replacing a shell and tube with a PHE. In all new process systems, provision for heat recovery should be considered from the start. The result is a system optimized for maximum economy savings.

PHEs are specifically configured to meet the duty, taking into account the product characteristics. Hot and cold fluids are directed between thin metal plates, corrugated to induce turbulence. The hot fluid transfers the heat to the cold stream, which is heated to a point where it requires little additional energy expenditure after the exchanger to reach the desired final temperature. In many applications, the fluids are the same product. Examples are raw and pasteurized milk, wet and dry crude oil, lean and rich amine. In other cases, one stream is product and the secondary stream is process water being preheated for use elsewhere in the plant, or cooled to be sent down the drain. Heat recovery is also possible with two different products or with uneven flows. Capturing the heat before it is lost is critical for major energy savings.

Heat recovery can be accomplished with different exchangers. However, PHEs can handle the requirements most efficiently and economically, and they are compact, low-weight, versatile and durable when compared to other exchangers. When the application is within the pressure and temperature limits of a PHE, it is often the best option.

A wide variety of plate materials are available; however the most common are 304SS, 316SS, titanium and hastelloy. Other exotic materials are also available to meet the application requirement. Common gasket materials are NBR and EPDM, with other specialty materials available. Welded plate pairs, a welded channel path with limited exposure to gasket material, can be ordered for fluid streams that have gasket compatibility problems.

Power House Recovery Payback

In an operation in the Northeast, a leading chemical producer installed two PHEs to handle power house heat recovery duties. Running 300 gallons per minute of 224 degrees F condensate against 400 gallons per minute of process water, slightly more than 5.5 million BTUs of heat are recovered per hour as the process water temperature is increased to 209 degrees F. It was estimated that the payback for this PHE was achieved in 483 hours. By passing the 187 degrees F condensate to the second PHE against 700 gallons per minute of 65 degrees F feed water, an additional 16.3 million BTU per hour of heat was recovered as the feed water was heated to nearly 112 degrees F, resulting in a payback in only 162 hours.

Geothermal System Recovery

In the upper Midwest, a PHE was installed in a geothermal heating system for municipal buildings. Since geothermal well water is corrosive, its heat is transferred via the PHE to the closed circuit system for space heating. The geothermal water pipes are cast iron, so the corrosion problem is confined to the heat exchanger. It is eliminated there by the use of 316 stainless steel plates.

Geothermal water is boosted to a pressure of 60 psig and enters the PHE at 165 degrees F. At its maximum flow of 500 gallons per minute, it heated 320 gallons per minute of closed circuit water from 100 degrees F to 130 degrees F. The outlet temperature of the closed circuit water is maintained by a pneumatic valve in the geothermal loop which regulates the amount of incoming hot water. The PHE response is almost instantaneous and temperatures can be controlled to an accuracy of 2 degrees F. Under normal conditions geothermal energy provides all the heat required at 8 million BTU/hr. If necessary, a boiler can also be employed in extreme conditions.

Whiskey Processing

At an east coast import whiskey plant, the spirit is first chilled to precipitate insolubles, then filtered and warmed to avoid bottling difficulties. It is processed at 2,500 gallons per hour, seven hours per day, in a three section PHE, which replaced batch processing in five large tanks. By using 80 percent heat recovery, chilling time was cut by more than 75 percent, reducing required refrigeration loads by 84 tons per hour and boosted production by 70 percent.

As the 90-degree-F product enters the regeneration section of the PHE, it exchanges heat with the outgoing product cold stream. The 90-degree-F product is cooled to 42 degrees F. After chilling to 30 degrees F in the second section of the PHE and leaving the PHE to filtration, it is passed back through the PHE in the regeneration section. It is heated to room temperature for bottling using hot water in trim heating section.  All three sections are designed into one frame, using what is called “grids” that contain auxiliary connections, making the unit compact and versatile.

The whole process is achieved in a PHE with little more than 10 square feet of floor space. The PHE is opened and cleaned once per year, the only necessary maintenance. The increase in productivity of approximately 7,000 gallons per day was accomplished without additional manpower.

Canning Cooling

A single PHE in a food plant is adding substantial savings by using incoming cold process water to reduce the temperature of used can cooling water. Spent can water at 2,350 gallons per minute is cooled from 95 degrees F to 83 degrees F using 600 gallons per minute of cold process water at 60 degrees F. The cold process water leaves the PHE at 85 degrees F ready to use without further heating. Heat savings by recuperation is 9 million BTU/hr, and the plant is in use 17 hours per day, 5 days per week and 48 weeks each year. Other benefits include a reduced BTU requirement for the cooling fan and lower evaporative water losses. The system paid for itself in three months.

Energy Savings Through Water Conservation

Although known as the ideal means for heating and cooling process streams, the PHE works equally well as a water conservation tool. The close temperature approach characteristic of the PHE permits the use of the cooling tower water for longer periods of the year. APV innovative engineering capabilities along with the PHE performance typify the reduced water consumption enjoyed by a chemical acid producer. In the past, the cost of process water was considered a minor part of the operation. Constantly increasing charges for city water and sewer services have altered that perspective.

The process required 340 gallons per minute of 70-degree-F water used to reduce the temperature of 625 gallons per minute of product from 190 degrees F to 180 degrees F. The company purchased close to a million gallons of water weekly, a significant expense. The PHE engineers calculated that a small PHE could easily replace the tubular unit being used. The solution was to remove about 3 million BTU/hr from 20 percent of the product flow, cooling only this amount to 138 degrees F, instead of the entire 625 gallons per minute.

After the PHE, the 138-degree-F slip stream was mixed with the remaining hot product to arrive at the desired overall temperature of 180 degrees F. Under this design, the cooling water requirement was reduced to 149 gallons per minute. This resulted in a 56 percent reduction in cooling water consumption. Payback of the exchanger occurred after just a few hours of operation.

Minimizing the Capital Cost and Justification With Payback

The installation of a heat recovery system does not have to involve a large capital outlay. The heat recovery system can consist of a little piping and a PHE or more elaborate heat recovery systems, depending upon the requirements of the application. As illustrated in the case studies, the payback for the system can often be realized within hours of operation.

Pumps & Systems, January 2011

Cheryl Shoemaker is a heat transfer specialist with SPX Flow Technology.